It's quite expected that plugins written for Ext3 won't work in Ext4 - plugins usually rely a lot on the inner workings of components that they change, and in Ext4 really a lot has changed.

It currently looks to me, that "my" FilterRow has to be pretty-much rewritten from scratch for Ext4. I'm still only in the early process of understanding how the whole Grid internally works in Ext4. So I really can not tell when will I take up this task.